Radiation current source



June 2, 1953 H. H. ROSSI 2,640,953

RADIATION CURRENT SOURCE Filed June 6, 1950 INVENTOR. HARALD H. ROSSIPatented June 2, 1953 the United States of America as represented by theUnited States Atomic Energy Commission Application June 6, 1950,=SerialNo. 166,489

"2 Claims.

The present invention relates to a variable source of minute currents.In the measurement of extremely small currents in the order of amperesasecondary sourceof very small current is often required. Accuratemeasurements of :such -.currents can be made, for example, with asensitive elect-rometer by balancing a known current against an unknowncurrent so 'that the electrometer does not deflect. It is'also sometimesdesirable to have a source of current the absolute value of which is notknown but which remains constant or which may be varied reproduceably.The present invention provides a source or current which remainsconstant or which may be varied reproduceably over extended periods-oftime. :By calibrating the source such as is provided according to thepresent invention with sources of 'knownintensity a source of knowncurrent intensity maybe produced.

.It-isaccordingly an objectof the present invention to provide avariable source of .minute currents which may be varied reproduceably.

It is another object of the present invention to provide an apparatuswhich can supply minute currents over a relatively wide range.

It is a further object of the present invention .to provide a method of(producing minute currents with high accuracyand reproducibility.

Other objects and advantages of the present invention will be partobvious and in part anointed out hereinafter.

,In ne of its broader aspects the objects of the present invention maybe achieved .by .pro-

viding an apparatus comprising a container for :enclosinga gasat a knownpressure, a source of charged particle radiation disposed in .said .con-

tainer, .aplurality-of ion collector plates in said containenatleastoneof said plates being adaptredto bemoved relatively to .an atleast partially shielded ion collector plate, said ion collector plate.being insulated from said movable .plate and from said container, andan .electrical conductor communicating between said collector plate andthe container exterior through an insulated port.

The subject apparatus is described with reference to theaccompanying.drawing, wherein:

B'i ure .1 .is a transversesectional view of the current source of thepresent invention taken aldngiline l--.I oiFigureZ, and

fieure 2 is a vertical section of the current source of Figure -1 takenon the line "22 of Figu e "1.

It "will be understood however that the .embodiment disclosed herein isgiven primarily for illustrative purposes and is not intended to beunderstood as limiting 'thescope of the present invention. Thereforesubstantial variations and substitutionsmaybemade in the elementsuof thedisclosed invention without departing from the soopethereof.

carrying out the method of the present invention in one of its broaderaspects, gas in a container is ionized by emanations from a radioactivesource, a potential is impressed on electrodes in thecontainer to causethe ions to move, a collector electrode is interposed in the path of themoving ions, and a movable shield is disposed adjacent said collectorelectrode to collect at leasta portion of the ions which move towardsaid collector electrode.

An apparatus for carrying out this method is described with reference tothe drawings.

The apparatus of this particular embodiment comprises generally aradioactive source 12 within acontainer 1.0. The source is ,kept at ahigh potential and the container at ground potential. The radiation fromthe source I? causes ionizationof gas in the container [0. Due to thehigh potential .betweensource l2 and container is ions are caused tomove in generally radial di rections within the container. In so movingthey impinge on collector plates I6, l8 and 12 causing a charge toaccumulate on the collector plates. If the charge is removed as fast asit is collected a relatively constant current will flow from thecollector plates.

The apparatus is described in greater detail with reference to theFigures 1 and 2. A container l0 encloses an ionization chamber. Thecontainer consists ofa-cylindrical tube 54 which forms the outer wall ofthe container and of a top .plate 56 and a bottom plate .58 which formthe 'topand bottom of the container. Within the container two insulatorrings 22 and 24 are po- .sitioned .at the bottom and top of thecontainer respectively. These insulated rings support a plurality ofioncollector plates I8 and 12. These collector plates are in the form ofradial segments of tubes extending around an arc of about 120 degrees.

Plates [8 and 12 are positioned in generally concentric relation to thetube 54 and the source of charged particle radiation 12. The shieldplate 16 isalso in the form of a radial segment of a tube extendingaround an arc of about 120.

.It isdisposed in the container on the concave side of ,plates 18 and 12but is eccentric in its relationtothe tube54 and source I2. The outerlongtltudinal edge of the plate I6 is preferabl bevelled to provide ablade edge at its outer surface. The plate l'6 is attached toandsupported at its upper end by'flange plate 66 and at itslower end by .asimilarplate 64. The flangeplates 6,4 and 66 are rotatably supportedin'the chamber by'their respective tubular extensions 61 and 65. Tube:65 extends through upper plate 56 and rests against its inner surfaceat shoulder '69. The

end oftube 6'l=thrusts against the inner surface of bottom-plate 58'andis held imposition-by an insulator l4 extending *through the tube core.The lower flange plate 64, the-shield plate It and as follows.

the upper flange plate 66 maybe rotated as a unit. The handle 2|] isattached to the externally extending portion of tube 65 in order toprovide means for rotating the shield plate I6 within the chamber. Thehandle 20 is held to the tube 65 by the set screw 2 I. Thus by rotationof the handle 20 the flange plates 64 and 66 are rotated and the shieldplate I6 may be disposed at any desired position within the chamber. Thebladed edge of shield plate I6 is brought close to the concave surfaceof collector plate I8 as the handle 20 is rotated. This arrangement ispreferred in order to minimize the change in capacitance between themembers It and I8 as the shield plate I6 is shifted. The change incapacitance is minimized with this arrangement because as is apparentfrom Figure l the bladed edge of the shield plate is relatively close tothe collector plate I8 and thus carries most of the capacitance existingbetween the two plates. As the shield plate It is shifted in itsposition with relation to plate I8 the bladed edge remains in closeproximity to the plate It and since it carries most of the capacitancelittle change in capacitance between the members results from shiftingof the plates.

Ionization of the gas in container I is produced upon radiation from aradioactive source I2. This source emitting preferably beta radiation ispositioned in a generally axial position with respect to the cylindricalchamber wall 54. A foil of silver containing approximately tenmilligrams of radium may be used and the foil may be enclosed as a stripwithin an axially extending glass tube I2. .sealed at one end andimbedded at the other into 'a metal cup 50 by a sealing composition orwax 52. The exterior surface of the glass tube is preferably madeconducting by coating with graphite or a similar conducting composition.It is desirable to employ an hermetically sealed container such as theglass tube l2 in order to prevent the radon gas, emitted from the radiumsource, from contaminating the chamber enclosed within the container 54.The electrically conducting external surface of the tube I2 iselectrically connected to a source of potential through the metal cup 59and rod I. R/od 5| is insulated from the bottom plate 58 of thecontainer Ill and from lower flange plate 64 by the insulator I4. Theinsulator sleeve I4 is threaded at its externally extending portion toreceive the locknut 60. By tightening the nut 60 against the outersurface of the plate 58, a shoulder 59 on the insulator sleeve I4 isurged into contact with the inner surface of the plate 58. A secondlocknut 62 may be threaded onto the externally extending portion of therod 5| to retain it in position in the insulator I4. By removal of thenuts 50 and 62, the tube I2 and rod 52 may be removed through the port68 and replaced as desired by a source of greater or lesser intensity.The internal end of the insulator I4 is fitted axially into the hole inthe lower flange plate 64 thereby providing an axis about which theplate may be rotated.

The operation of the current source is generally The high potential isapplied to the conducting surface of the tube I2. The container I0 isconnected to ground. This grounds the shield plate I6 which iselectrically connected to the container II]. The collector plate I8 isnot grounded but is insulated from the container II] by the upper andlower insulators 24 and 22 respectively. Theradioactive source emitsbeta ti lesnin al ld ect q eerieral yre i lly fr The tube is preferably4 the tube I2. Ionization of the gas within the container IU' provides asource 'ofcharged particles which can carry a charge between conductingsurface of tube I2 and the container wall 54. The high voltage appliedbetween the conducting surface of the rod I2 and the container wall 54induces radial motion of the ions and thus causes a charge to collect onplate I8. Charge collected on plate I8 may be removed as a minutecurrent through externally communicating conductor 26. The position ofthe shield plate I6 determines how many of the charged particles canimpinge on the plate member I8 because the plate I6 can be locatedbetween the plate I8 and the source I2 where it intercepts ionsaccelerated toward plate I8. Thus, when a large number of positivelycharged particles are formed in the region ambient to the source I2 anda high potential' is applied to the source tube I2 while the containerIt is grounded, positive ions will proceed generally radially from thesource I2 toward the container wall 54. If the plate It is positioned toshield part of the surface of the plate I8 from the radially movingions, a portion of the ions will impinge on the plate I5 and anotherportion will impinge on the plate I8. The number of ions impinging onplate I8 may be changed by changing the portion of plate I8 shielded byplate IE. It is possible to change the ion current in still a third way.If the position of the plate I6 and the intensity of the radiation fromthe source I2 are kept constant and the container It is hermeticallysealed by plugging port 68, an ion current will flow to the plate memberI8 which is roughly proportional to the pressure of the gas enclosedwithin the container I0.

It is possible to provide minute currents over a very wide intensityrange by presetting three parameters. That is, the intensity of the ioncurrent may be changed by changing (1) the intensity of the radiationsource, (2) pressure of the gas in the container I0 and (3) the positionof the plate i6 relative to the plate I8. By providing an hermeticallysealed chamber within the container I B and a source of known intensity,

adjustment of pressure, and then to vary the current entirely byrotating the handle 20. The handle position for various source currentsmay be calibrated so that by adjusting the position of the handle 20 acertain current will flow. This is true if the voltage developed on thecollector I8 is small with respect to the voltage impressed between tubeI2 and the container I0.

As previously stated. the embodiment disclosed is illustrative of thepresent invention. The in= vention may be embodied in many alternatedevices without departing from the scope thereof. For example, a rod 'ofstrontium metal or other high energy beta ray emitter having a long halflife might be used. Sucha rod might be prepared, forv example, byinserting it into a nuclear reactor for a sufiicient length of time togive it the desired activity. This type of .rod source does not emitradon.

The configurationof the disclosed embodiment, wherein a movable plate ispositioned eccentrically to the tubular arrangement of the apparatus, isused principally because there is a danger .of a-troublesome capacitancebeingdeveloped between the two plate members I6 'and lfit v The knifingof one edge of the plate l6 helps to minimize the effects of acapacitance development as explained above. The arrangement disclosed issatisfactory for producing a constant current over an extended period oftime. However, changing of the current delivered by shifting the vane l6may result in changing the capacitance between the plate members. Achange in the capacitance results in the production of a pulse currentor current kick. It is possible to avoid this latter result by attachinga second chamber having exactly the dimensions of the first butcontaining a source l2 which is not radioactive. The second chamber isattached so that the shifting of its plates compensates for the changein capacitance between the plates of the first chamber. Such a chambermay be positioned, as indicated in phantom view at the top of Figure 2,as a mirror image of the apparatus described in the embodiment. In thiscompensating arrangement the movement of the calibrated handle 2ll2llresults in the rotation of a plate H5 in the radioactive portion of theapparatus and a plate 16' in the non-radioactive mirror image of theapparatus. External brackets 10' may be used to join the upper and lowerchambers 10 and H], to prevent their relative rotation when the handle20 is rotated.

It is also possible to provide a greater range of available currentintensities from such a source by inclusion of auxiliary plate memberssuch as 1'2 having shorter lengths than plate I8 but disposed in thesame circle as plate 18 within the container ID. The shield plate 16 maybe used to partially shield the auxiliary plates or the collector plateIt alternately. A greater range of currents is provided because theshorter collector plates such as 12 collect only a fraction of thenumber of ions which the longer plate IE will collect for the sameangular exposure. That is. if, half the plate I8 is shielded and halfexposed it will collect a certain amount of ions. If half of a platesuch as 12 is shielded and half is exposed and the plate is onlyone-tenth the length of plate I8 approximately one-tenth of the numberof ions will be collected on the half of plate 12 which is exposed aswould be collected on onehalf of the plate 18 which is exposed. Thus, ifthe auxiliary plate 12 is approximately one-tenth the length of theplate l8 the number of ions collected for a given angle of exposure isreduced by a factor of ten. A separate insulated lead M is necessary foreach of the plates and the plates must be insulated from each otherwhere it is desired to collect ion current from the plates individually.

It is apparent from the foregoing that the radiation current sourceprovided according to the present invention is an extremely useful andsimple device for supplying minute currents. The apparatus may also beconsidered a variable resistance since it is possible to change thecurrent flowing without changing the potential drop in the apparatus.The accuracy and reproducibility of the current supplied by the presentapparatus under constant pressure and temperature conditions has beendemonstrated to be better than 1 percent.

Since many embodiments might be made of the present invention and sincemany changes might be made in the embodiment described, it is to beunderstood that the foregoing description is to be interpreted asillustrative only and not in a limiting sense.

I claim:

1. Apparatus for supplying minute currents over a wide range whichcomprises in combination a substantially cylindrical, gas filled,hermetically scalable chamber, a source of charged particle radiationaxially disposed in said chamber, a first arcuate electrode fixedlymounted within said chamber and electrically insulated therefrom, saidfirst electrode extending substantially the length of said chamber, asecond electrically insulated arcuate electrode fixedly mounted withinsaid chamber and diametrically opposed to said first electrode, saidsecond electrode being substantially shorter than the length of saidcham her, a third arcuate electrode rotatably mounted within saidchamber and electrically connected thereto, said third electrode beingrotatable between said radioactive source and said first and secondelectrodes, means external to said chamber for rotating said thirdelectrode, and means for impressing a high potential between saidradioactive source and said container whereby radiation emitted fromsaid source produces ions in said chamber which are collected by saidfirst and second electrodes and are intercepted by means of said thirdelectrode.

2. Apparatus for supplying minute currents over a Wide range whichcomprises in combination a substantially cylindrical, gas filled,hermetically scalable chamber, a cylindrical radioactive source holderaxially disposed in said chamber, said source holder having anelectrically conductive coating, a radioactive source mounted withinsaid source holder, a first cylindrical tubular segment fixedly mountedwithin said chamber and electrically insulated therefrom, said firstsegment extending substantially the length of said chamber, a secondelectrically insulated cylindrical tubular segment fixedly mountedwithin said chamber and diametrically opposed to said first segment,said second segment being substantially shorter than the length of thechamber, a third cylindrical tubular segment mounted within said chamberelectrically connected thereto and rotatable about an axis parallel tosaid radioactive source holder, said third segment being closer to saidsource holder than said other segments, means for rotating said thirdsegment external to said chamber, and means for impressing a highpotential between the electrically conductive coating on saidradioactive source holder and said chamber whereby radiation emittedfrom said source produces ions in said chamber which are collected bysaid first and second segments and are intercepted by means of saidthird segment.

HARALD H. ROSSI.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Re. 22,982 Foulkes Mar. 9, 1948 2,494,641 Anderson Jan. 1'7,1950 2,497,213 Downing Feb. 14, 1950 2,517,120 Linder Aug. 1, 1950 OTHERREFERENCES The Electrician, October 31, 1924, page 497.

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Nuclear Electrostatic Generator, Linder, Physilcsallgeview, vol. 71, #2,pages 129, 130, January

